Model-based calibration of an adaptive interferometric setup

The measurement of aspheres and freeform surfaces is one of the main challenges in production control. Amongst others, interferometry provides a high accuracy at specular surfaces. To measure aspherical lenses, it is necessary to adapt or partially adapt the wavefront to the surface under test. Recently, we have proposed a non-null interferometric setup with two diffractive elements for this purpose (DGaO, A23, 2014). By shifting these two elements in both lateral directions, the combination of their phases results in a linear respectively quadratic dependency. This yields a variable defocus and astigmatism and can adapt the wavefront to constant focus plane. To use this system for measurements, the main challenge is its calibration. While different defocus positions can be calibrated by standard methods like three positon test or random ball test, the astigmatism must be handled as a non-null system. Therefore we use a black box model recently introduced for the tilted wave interferometer of our institute. To reach calibration data, we solve an inverse problem deduced by a black box model of the illumination and imaging optics separately. We will present the functionality of this calibration and some simulation results.